The present invention relates to a bonded part comprising a rubber member bonded to a second substrate, and to a rubber-to-metal bonded part, and to a torsional vibration damper, and to a process for directly bonding rubber to at least a second substrate which may be a metal substrate, and to a method of bonding an intervening rubber member, which may optionally be applied under compression, between two metal members, such as in the manufacture of crankshaft torsional vibration dampers.
High strength bonding of rubber to substrates and particularly to metal substrates is desirable for numerous applications, including in the manufacture of rubber composite articles characterized by high and/or dynamic loading or extreme environmental conditions, e.g., tires, belts, rolls, seals and hose; and in those applications involving or calling for some level of vibration isolation and/or shock damping, e.g. vibration isolators such as engine mounts, vibration isolation mounts, vibration dampers, couplings, suspension bushings and transmission and axle seals.
A wide variety of methods have historically been employed to address one or another aspect of rubber-to-metal bonding, including improving adhesive strength, controlling the rubber compression or shrinkage level and/or increasing production efficiency, etc. In general, particularly in the area of vibration isolators and/or shock dampers, wherein an annular cured rubber member is disposed or sandwiched between two outer metal substrates, a limited level of mechanical bonding can be achieved by compressing the rubber member between outer metal members and relying on frictional forces between the rubber and metal surfaces. High strength rubber-to-metal bonding however is generally achieved through adhesive bonding of the fully vulcanized rubber member placed between the outer metal members through the action of one or more adhesives applied at the rubber-to-metal interface (hereafter, “post-vulcanization bonding”). An advantage of post-vulcanization bonding in the construction of parts wherein a rubber member is disposed between two outer metal surfaces is that since the rubber member is fully cured prior to its placement between the metal surfaces, it does not exhibit significant shrinkage and thereby resides under tension between the metal surfaces upon exposure to temperatures below its vulcanization temperature.
Alternatively, un-cured elastomeric material is introduced between the outer metal members to which a rubber-to-metal adhesive has been applied, and the elastomeric material is then fully cured in contact with the adhesive-coated metal substrate (hereafter, “vulcanization bonding”). A high degree of process control is required in the practice of this method in order to provide a homogeneous and consistent product. Moreover, dampers must be assembled relatively quickly after rubber mixing according to this method, which reduces production flexibility, and therefore, production efficiency. An additional disadvantage of conventional vulcanization bonding techniques in the construction of parts wherein a rubber member is disposed between two outer metal substrates is that since the rubber member is fully cured while in contact with each of the metal surfaces, it tends to exhibit at least some degree of shrinkage after curing, and thus resides under tension between the metal surfaces almost immediately as the exposure temperature falls below the vulcanization temperature, with adverse impact on the durability of the article.
In both of these methods, compression forces have also optionally been applied to provide further stabilization of rubber-to-metal engagement or to eliminate the tension resulting from shrinkage of the rubber.
In general, in each of the three conventional methods of press molding rubber or rubber-to-metal bonded assemblies exemplified by crankshaft torsional vibration dampers according to either vulcanization- or post-vulcanization bonding techniques, i.e., compression molding, transfer molding and injection molding, the problems associated with rubber-to-metal adhesives applied at the rubber-to-metal interface are essentially the same. First is the environmental concern; most such adhesives contain toxic constituents and are thus difficult and costly to handle, to store and to dispose of. Prior to the application of the rubber-to-metal adhesive, the relevant metal surface must moreover generally undergo intensive surface cleaning and preparation to ensure adequate bond strength. Furthermore, due to their typically volatile nature, the rubber-to-metal adhesive composition may sublimate or volatilize at vulcanization temperatures prior to the point at which adequate contact between the metal and the rubber is achieved, thereby decreasing the adhesive's efficiency, potentially causing fumes at the press and/or resulting in mold fouling. In addition, in vulcanization bonding processes there is the problem of “mold sweeping”, whereby as molten rubber enters the mold cavities prior to curing, it flows across the adhesive-coated metal, tending to sweep along with it at least a portion of the adhesive, thus further reducing its efficiency.
U.S. Pat. No. 4,889,578 to Kei et al. describes a process for making a rubber vibration insulator including the steps of adhering, by vulcanization in combination with a metal adhesive at the rubber-to-metal interface, an un-vulcanized rubber layer to the outer surface of an inner metal fitting; adhering by vulcanization in combination with a metal adhesive at the other rubber-to-metal interface, another un-vulcanized rubber layer to the inner surface of an outer shell metal fitting; applying a halogen compound solution to the opposite, non-bonded surfaces of both of the rubber layers; press-fitting the inner metal fitting having the rubber layer to the outer shell metal fitting having the rubber layer such that the two rubber layers form a rubber-to-rubber interface, using a lubricant or a lubricating adhesive, and effecting adhesion between the vulcanized rubber layers through heating the above described mutually fitted bodies.
This process presents several drawbacks. In particular, the utilization of a halogen compound e.g., chlorinated or brominated polymers and sodium hypochlorite, or chlorinated cyanuric acid solution as a pretreatment agent, is still required to bond the adjacent vulcanized rubber surfaces. Moreover, the process is characterized by a plurality of labor steps; each of which introduces incremental cost increase to the process. In addition, the process relies nonetheless on the utilization of a rubber-to-metal adhesive on the metal surface prior to application of the rubber thereto in order to achieve satisfactory adhesion of the rubber to the metal.